Method for the production of an ordered porous structure from an aluminium substrate

ABSTRACT

A method for making a porous structure, includes producing, by anodization of an aluminum substrate, an outer surface layer ( 3 ), part of the thickness of which is formed by an ordered porous structure ( 7 ), characterized in that it includes an anodization step on a smooth aluminum substrate for a duration sufficient for obtaining a thickness of ordered porous structure ( 7 ). The method further includes removing by mechanical machining a portion of the thickness of the layer ( 3 ) formed by anodization, the thickness portion extending from the outer surface of the layer ( 3 ) formed by anodization, while maintaining an ordered porous structure ( 7 ) with a non-zero thickness, so that the ordered porous structure defines the free outer surface of the residual layer.

The invention relates to a method for the production of an orderedporous structure from an aluminium substrate.

Throughout the entire text, a porous structure is called “ordered” if ithas pores in the form of rectilinear channels of the same transversecross-section (shape and dimensions) which are parallel and adjacent ina radial plane and uniformly distributed in the radial plane.

In addition, throughout the entire text the aluminium piece and theanodic structures resulting from anodization of the said aluminium pieceare orientated according to their two opposite faces, a first face, theso-called outer face, in contact with the electrolyte solution and asecond face, the so-called substrate face, which is not in contact withthe electrolytic solution.

In addition, throughout the entire text alumina is understood as meaningthe general term covering oxidized forms of aluminium, that is to sayaluminium oxides, aluminium hydroxides and also aluminium oxyhydroxides.

Many electronic, mechanical, biotechnological or chemical systems aretending towards extreme miniaturization, opening up a vast field ofapplications in domains as varied as medicine, aeronautics, space,electronics, information technology or photonics. For this purpose,control of the structure of materials, of dimensions and of theregularity of their ultrastructures is becoming essential in order toreduce the dimensions of these systems, to increase the ratio betweenthe specific surface area and the total volume of the sample and/or toobtain specific physical phenomena.

With this aim, it is known to realize, by anodization of aluminium metalsubstrates, ordered porous structures based on the chemical elementaluminium, the surface of which extends over several μm². These porousstructures, also called porous anodic films, can be used as a support oras a matrix for original applications, such as nanofiltration, or alsothe realization of functional elements of nanometre dimension, such asnanocontacts, nanofilaments and nanotubes. The improvement in thetechnical performances of these materials, the ultrastructures of whichare of meso- or nanometre dimension, is attributed directly totechnological advances allowing the realization of porous anodic filmsof large dimension and controlled thickness.

The growth of a porous structure in the course of anodization of analuminium substrate is guided by a complex method involving anequilibrium between on the one hand an oxidation reaction of thealuminium into oxidized, hydroxylated or also oxyhydroxylated aluminiumderivatives and on the other hand a dissolving reaction of this aluminaformed. It is thus known that the formation of a porous structureresults from the equilibrium, depending on the anodization operatingconditions, between the respective contributions of these twoantagonistic chemical reactions. The publication Masuda H., Yada K. etOsaka A., (1998), Jpn. J. Appl. Phys., 37, 1340-1342 “Self-ordering ofcell configuration of anodic porous alumina with large-size pores inphosphoric acid solution” describes a method for obtaining a porousstructure comprising several treatments, that is to say an anodizationtreatment for a duration varying from 0.5 h to 16 h on anon-pretexturized aluminium substrate in a phosphoric acid solution at aconcentration of 0.3 mol/l under a voltage of 195 V, a chemicaldissolution treatment of the residual aluminium substrate by means of asaturated HgCl₂ solution, a chemical dissolution treatment of thebarrier layer, also called compact layer, by means of a phosphoric acidsolution of 10% strength by weight, and finally a chemical treatment forenlarging the diameter of the pores of the porous structure by means ofa phosphoric acid solution.

This document thus describes a reference method for the production of aporous structure based on aluminium by means of several successivetreatments, all of which are of a chemical or electrochemical nature.This document demonstrates the state of the surface of the substrateface of the porous structure after removal of the residual substrate andof the barrier layer. It does not describe the state of the porousstructure within its thickness, in particular at the outer face.

It is furthermore known that it is possible to obtain an ordered porousstructure by anodization of an aluminium substrate having on its outersurface a plurality of concavities of the same shape and regularlydistributed. Such an impression can be obtained by nanoindentation ofthe aluminium substrate, for example by applying to and pressing on thealuminium substrate a hard matrix, in particular of silicon carbide,having a plurality of convexities. However, this nanoindentation stageis technologically very difficult to carry out because of the technicaldifficulties in realizing the silicon carbide matrix having a pluralityof convexities on meso- and nanometre scales. This stage of realizationof a matrix is consequently an expensive stage.

Another known method allowing a plurality of concavities of the sameshape and regularly distributed to be obtained on the surface of thealuminium or aluminium alloy substrate is called “double anodization”.In this method of “double anodization”, a first anodization stage allowsthe formation of a plurality of concavities at the interface of theinitially smooth aluminium substrate and the porous structure resultingfrom this anodization. Complete dissolution, by a chemical route, of theporous structure resulting from the anodization then reveals theplurality of underlying concavities. These concave impressions thenserve to guide the growth of an ordered porous structure during a secondanodization stage. This method of “double anodization” takes a long timeto carry out because of the duplication of the anodization stage. Thismethod moreover necessitates a stage of chemical dissolution of theporous structure resulting from the first anodization, which isdifficult to carry out and consequently is not very, if at all,compatible with use on an industrial scale. This method furthermorenecessitates the use of toxic chemical products during the dissolutionstage, such as derivatives of chromium, in particular of chromium(VI).Finally, the thickness of the porous structure produced at the end ofthe initial anodization treatment on the aluminium substrate and thendissolved by the chemical treatment is not upgraded.

Other methods have been proposed to perfect the development of anordered porous structure, but without succeeding in avoiding carryingout a “double anodization” with intermediate removal of a structureformed by a first anodization. EP 1715085 thus proposes a method inwhich the chemical dissolution treatment is replaced by anelectrochemical treatment, leading to separation of the residualaluminium substrate and the entirety of the structure resulting from thefirst anodization. Here also, this method takes a long time to carryout, and is relatively complex, expensive and not very compatible withuse on an industrial scale.

Thus, to date, in order to obtain an ordered porous structure havingpores in the form of rectilinear channels of the same transversecross-section (shape and dimensions) which are parallel and adjacent ina radial plane and uniformly distributed in the radial plane, it wasalways considered necessary to carry out a pretexturization of thealuminium substrate, either by nanoindentation of the said aluminiumsubstrate or by a first anodization, followed by a chemicaldissolution/electrochemical separation.

In this context, the object of the invention is to lessen the effect ofthese disadvantages by proposing a production method for an orderedporous structure by anodization of a smooth aluminium or aluminium alloysubstrate which avoids resorting to a double anodization and which nolonger necessitates carrying out a prior stage of mechanicalnanoindentation of the aluminium or aluminium alloy substrate.

The object of the invention more particularly is to propose a method forthe production of an ordered porous structure by anodization which issimple, rapid, inexpensive and environment-friendly, and which iscompatible with use on an industrial scale.

The object of the invention more particularly is to propose a methodallowing an ordered porous structure to be obtained which is of highquality and homogeneous throughout its entire thickness and in which theshape, the diameter of the pores and the organization of the pores areperfectly controlled.

The object of the invention more particularly is to propose a methodallowing an ordered porous structure to be obtained, which can have ahigh thickness—in particular greater than 50 μm.

The object of the invention is also to propose a method for theproduction of an ordered porous structure which does not necessitate theuse of toxic chemical compounds, such as derivatives of chromium, inparticular of chromium(VI).

The invention thus relates to a method for the production of a porousstructure in which an outer surface layer comprising an ordered porousstructure is produced by anodization of an aluminium substrate,characterized in that:

-   -   an anodization treatment is carried out on a smooth aluminium        substrate with a duration sufficient to allow at least a        thickness of ordered porous structure to be obtained,    -   a part of the thickness of the said layer formed by anodization        is then removed by mechanical machining, this part of the        thickness extending from the outer surface of the said layer        formed by anodization, while maintaining a non-zero thickness of        the ordered porous structure and in a manner such that this        ordered porous structure forms the free outer surface of the        residual layer.

The porous structure obtained by a simple anodization of a smoothaluminium substrate has on the side of its outer face a thickness ofimperfectly ordered porous structure—that is to say which does not havepores in the form of rectilinear channels of the same transversecross-section (shape and dimensions) which are parallel and adjacent ina radial plane and uniformly distributed in the radial plane. However,if the duration of the anodization is sufficiently long, the porousstructure also has, underlying this imperfectly ordered porousstructure, a perfectly ordered porous structure, that is to say havingpores in the form of rectilinear channels of the same transversecross-section (shape and dimensions) which are parallel and adjacent ina radial plane and uniformly distributed in the radial plane.

Thus, merely the act of carrying out directly an anodization on a smoothaluminium substrate, that is to say having a arithmetic roughness ofless than 5 nm and consequently not having a plurality of concavities asthe result either of a prior anodization—double anodization method—or ofa mechanical nanoindentation stage, allows in reality, if the durationof the anodization is sufficiently long, a thickness of ordered porousstructure to be obtained underneath an imperfectly ordered porous layerextending into the surface. According to the invention, the thicknesscorresponding to this imperfectly ordered layer is removed by mechanicalmachining in a manner such that the pores of the ordered porousstructure emerge in the surface.

In particular, in a method according to the invention, an anodizationtreatment is carried out from a substrate formed by an aluminium alloyof series 1XXX, for example aluminium alloy 1050A, or also refinedaluminium, in particular chosen from the group formed by aluminium 4Nand aluminium 5N.

In a method according to the invention, the outer surface layercomprising at least a thickness of porous structure is obtained after aduration of the anodization which depends on the speed of growth of thesaid outer surface layer. The speed of growth of the outer surface layerdepends here on the operating conditions chosen for realizing thephysical properties of the porous structure.

The method according to the invention thus allows a porous structurehaving a non-zero thickness of an ordered porous structure to berealized rapidly and easily in a single anodization and withoutnecessitating either a subsequent chemical treatment of selectivedissolution or an electrochemical separation. This ordered porousstructure has an open porosity at least on one of its faces, theso-called outer face, and a controlled thickness of ordered porousstructure on the micrometre scale.

That being so, according to a possible variant of the production methodaccording to the invention, nothing prevents the outer surface layerfrom being realized such that it comprises a plurality of superimposedthicknesses of ordered porous structures. These various thicknesses areobtained in particular by an anodization treatment comprising aplurality of successive anodization stages, none of the said anodizationtreatment stages being followed by a treatment by selective chemicaldissolution or electrochemical separation of a part of the thickness ofthe layer formed by anodization. In this variant, the variousanodization treatment stages are carried out under anodizationconditions in which at least one of the anodization parameters chosenfrom the group formed by the anodization voltage, the temperature of theanodization solution, the chemical composition of the anodizationsolution and the anodization current density is modified between twosuccessive anodization stages.

In a method according to the invention, any known method for removal ofmaterial by mechanical machining can be used to carry out the removal ofa part of the thickness of the said layer formed by anodization, thispart of the thickness extending from the outer surface of the saidlayer, while maintaining at least a non-zero thickness of ordered porousstructure and in a manner such that this ordered porous structure formsthe free outer surface of the residual layer.

In a method according to the invention, mechanical machining isunderstood as meaning any method suitable for superficial removal ofparticles of material. In a method according to the invention, theseparticle of material removed by mechanical machining are particles inthe solid state. However, in a method according to the invention, theparticles of material removed by mechanical machining can be in thegaseous state.

In a method according to the invention, any known method for removal ofmaterial by mechanical machining can be used, excluding treatments bychemical attack, in particular chemical treatments on the layer formedby anodization with a solution capable of penetration by capillarityinto the pores of the said porous layer and of modification of the shapeand size of the pores of the porous structure.

Such a mechanical machining can be carried out in a single stage, or onthe other hand by a plurality of successive stages. Such a mechanicalmachining can also be carried out with a single mechanical machiningtechnique used during the various machining stages, or on the other handby using a plurality of mechanical machining techniques duringsuccessive mechanical machining stages.

Such a mechanical machining according to the invention can be carriedout in particular by ionic polishing using an ion flux, in particular aprecision ion polishing system (PIPS), or also using the wide andhigh-energy primary beam of a secondary ion mass spectrometer (SIMS).

In particular, in a method for mechanical machining by removal ofmaterial by PIPS, the anodized outer surface layer is subjected forseveral hours, in particular between 1 h and 20 h, especially between 3and 6 h, to at least a beam of accelerated argon ions under a voltage ofbetween 1 keV and 6 keV, in particular of the order of 5 keV, under asecondary vacuum of the order of 1.31·10⁻³ Pa.

Advantageously and according to the invention, the said part of thethickness is removed by mechanical abrasion, that is to say by dynamicsolid/solid friction, by means of a movable abrasive solid tool, whichis applied to the outer surface of the porous layer formed byanodization, a pressure being exerted on the said movable abrasive solidtool.

Advantageously and according to the invention, this mechanical abrasionis carried out in a manner such that an ordered porous structure ofwhich the outer surface is flat is obtained.

More particularly, the said part of the thickness is removed by amechanical machining treatment, in particular by mechanical abrasion,which affects only the outer surface of the porous layer formed byanodization, and which does not affect, in the thickness of the porousstructure, either the diameter of the pores of the ordered porousstructure or the shape of the said pores revealed in the course of theabrasive treatment. This treatment by mechanical abrasion differs from atreatment by chemical dissolution, which necessarily affects not onlythe thickness of the porous layer formed by anodization, but also theshape and the diameter of the pores of the said layer.

Advantageously and according to the invention, the mechanical abrasionis carried out by means of a piece of fabric—in particular a piece offelt—impregnated with a suspension, so-called abrasive suspension, of apowder in an aqueous phase, the said powder comprising at least amineral chosen from the group of abrasive minerals consisting of diamondand ceramics—in particular corundum.

The use of a piece of fabric impregnated with an abrasive suspensionaccording to the invention allows a regular abrasion and a high finenessto be obtained. Furthermore, it also allows permanent wetting of thesurface of the porous layer obtained by anodization and maintaining ofthe temperature thereof, even in the course of mechanical abrasion. Itthus avoids deterioration of the porous anodic structure in the courseof the said abrasion.

The choice of abrasive mineral also allows the hardness of the saidmineral to be selected in a manner such that the speed of abrasion ofthe porous structure is controlled. At all events, the hardness of theabrasive mineral contained in the abrasive suspension is greater thanthe hardness of the porous layer, the composition of which is based onalumina, in particular oxidized, hydroxylated and/or oxyhydroxylatedaluminium derivatives.

Advantageously and according to the invention, the mechanical abrasionis carried out in a single stage, or on the other hand by a plurality ofsuccessive abrasion stages, each of the said successive abrasion stagesbeing carried out by means of an abrasive suspension, the abrasivesuspensions of each of the successive abrasion stages being chosen in amanner such that a granulometry which decreases from one stage to theother is achieved.

Advantageously and according to the invention, the mechanical abrasionis carried out by a succession of abrasion stages from a less fine andmore rapid abrasion to a finer and slower abrasion. The choice of thesize of the particles of the mineral powder allows both the speed of theabrasion of the porous structure and the quality of the finish of thesurface of the porous structure to be controlled.

The succession of abrasion stages carried out by means of abrasivesuspensions of decreasing granulometry also allows the abrasion time tobe reduced while imparting to the surface of the porous structure a lowroughness and an excellent finish.

Advantageously and according to the invention, each abrasion stage ofthe plurality of successive abrasion stages is carried out by means of apiece of fabric impregnated with an abrasive suspension, the said pieceof fabric being applied to the surface of a rigid support chosen fromthe group formed by a vibrating support and a rotating support.

In particular, each abrasion stage of the plurality of successiveabrasion stages is carried out by means of a piece of fabric impregnatedwith an abrasive suspension, the said piece of fabric being applied tothe surface of a rigid support chosen from the group formed by avibrating support and a rotating support, the smallest dimension of thepiece of fabric and of the rigid support being greater than the largestdimension of the outer surface layer.

Advantageously and according to the invention, each abrasion stage ofthe plurality of successive abrasion stages is carried out by means of arotating support having a speed of rotation of less than 30 rad/s, inparticular between 2 rad/s and 20 rad/s. The pressure applied to thesurface of the porous layer in the course of the mechanical abrasion isin particular between 1 kPa and 50 kPa.

Advantageously and according to the invention, the mechanical abrasionis carried out by a first abrasion stage by means of a piece of feltimpregnated with a suspension of diamond, the mean granulometry of whichis between 0.8 μm and 1.5 μm, in particular of the order of 1 μm, and bya second abrasion stage by means of a piece of felt impregnated with asuspension of diamond, the mean granulometry of which is between 0.2 μmand 0.4 μm, in particular of the order of 0.25 μm.

Advantageously and according to the invention, the total duration of themechanical abrasion is less than 30 min, in particular between 10 minand 20 min. This duration in practice allows the total thickness of thenon-ordered porous layer formed in the outer surface during theanodization to be removed.

In a variant of a method according to the invention, it is possible tocarry out a single mechanical abrasion stage of a duration of the orderof 20 min by means of a piece of felt impregnated with a suspension ofdiamond, the mean granulometry of which is close to 0.25 μm.

According to another variant of a method according to the invention, itis possible to carry out three successive mechanical abrasion stages.Three mechanical abrasion stages are carried out successively, each ofthese three stages being of a duration of the order of 10 min. The firstabrasion stage is carried out by means of a piece of felt impregnatedwith a suspension of diamond, the mean granulometry of which is close to1 μm, the second stage is then carried out by means of a piece of feltimpregnated with a suspension of diamond, the mean granulometry of whichis close to 0.25 μm, and finally the third stage is carried out by meansof a piece of felt impregnated with a suspension of diamond, the meangranulometry of which is close to 0.10 μm.

There is nothing to prevent the mechanical abrasion from being carriedout by more than three successive stages. There is moreover nothing toprevent the mechanical machining stages from being carried out withdistinct machining techniques, for example chosen from the PIPS, SIMSand abovementioned abrasion techniques.

Advantageously and according to the invention, a thickness of the outersurface layer of between 15 μm and 25 μm—in particular of the order offrom 17 μm to 20 μm—is removed. This thickness represents at least thethickness of the imperfectly ordered porous structure extending from theouter surface of the layer formed by anodization.

Advantageously and according to the invention, an anodization treatmentis carried out on a smooth aluminium substrate with a duration suitablefor obtaining an outer surface layer formed by anodization having atotal thickness of between 25 μm and 300 μm, in particular between 100μm and 200 μm.

Advantageously and according to the invention, a single anodizationtreatment is carried out on a smooth aluminium substrate, the saidtreatment having a duration of between 1 h and 12 h, in particular ofthe order of 4 h.

A method according to the invention thus comprises carrying out a singleanodization treatment comprising at least one anodization stage and thena stage of removal of the part of the thickness of the outer surfacelayer of which the porous structure is imperfectly ordered. According toa method according to the invention, the anodization stage marking theend of the anodization treatment is immediately followed by a treatmentby mechanical machining, in particular by mechanical abrasion.

Advantageously and according to the invention, a single anodizationtreatment is carried out on a smooth aluminium substrate over a durationsuitable for the thickness of the ordered porous structure formed byanodization to be between 1 μm and 150 μm, in particular between 50 μmand 150 μm.

Advantageously and according to the invention, the anodization iscarried out in an aqueous solution of electrolyte chosen from the groupformed by aqueous solutions of acids—in particular sulfuric acid, amixture of sulfuric acid and boric acid, oxalic acid, phosphoric acid,malonic acid, tartaric acid and citric acid.

Furthermore, advantageously and according to the invention, theanodization is carried out in an aqueous solution of electrolyte, thecomposition of which is suitable for providing an ordered porousstructure, the pores of which have a diameter of between 10 nm and 500nm, in particular between 100 nm and 200 nm.

In addition, advantageously and according to the invention, theanodization is carried out at a temperature of between −2° C. and +2°C.—in particular of the order of −1.5° C.

Advantageously and according to the invention, the anodization iscarried out under a voltage of between 19 V and 240 V—in particularbetween 125 V and 195 V, with an aqueous solution containing phosphoricacid as the electrolyte.

In particular, in a method according to the invention, at least oneanodization is carried out in a single stage, or in a combination ofimmediately successive anodization stages, and then a mechanicalabrasion treatment is carried out. A porous structure comprising atleast one thickness of ordered porous structure is obtained. Thus, in amethod according to the invention, it is not necessary to carry outother anodization treatment after having removed, in particular bymechanical abrasion, the part of the thickness of non-ordered structureof the layer formed by the first anodization. Consequently, in a methodaccording to the invention, a single treatment by anodization of thealuminium substrate is carried out. After the stage of removal bymechanical machining of the non-ordered structure, in particular bymechanical abrasion, other subsequent treatments can possibly be carriedout, but it is not necessary to carry out either chemical orelectrochemical dissolution or a new treatment by anodization.

Advantageously and according to the invention, immediately after havingremoved the said part of the thickness, the non-oxidized aluminiumsubstrate and a part of the non-porous thickness of the said layer areremoved to preserve only the ordered porous structure.

Advantageously and according to the invention, a chemical treatment isthen carried out on the ordered porous structure which is suitable forincreasing the diameter of the pores of the said porous structure.

Such a chemical treatment is particularly suitable for partly dissolvingthe dividing wall of the pores from the face of the said dividing wallwhich is facing the pore and in the direction of the internal part ofthe dividing wall. The inventor has found that the chemical compositionof the layer of material constituting the said dividing wall variesaccording to the radial axis of the pores. The chemical composition ofthe face of the layer of material constituting the dividing wall facingthe pore is a mixture based on oxidized, hydroxylated and/oroxyhydroxylated aluminium and comprising up to 20% of compoundsresulting from the electrolytic solution used for the anodization. Inreturn, the internal part of the said dividing wall is composedessentially of oxides, hydroxides and/or oxyhydroxides of aluminium.

The choice of the duration of this treatment of opening the pores andalso the nature of the chemical agent chosen for this treatment allowsthe thickness of the layer of material dissolved inside the pores to becontrolled and, consequently, the final diameter of the pores of theordered porous structure to be determined.

The invention also relates to a method for the production of a porousstructure, characterized by a combination of all or some of thecharacteristics mentioned above or below.

Other objects, characteristics and advantages of the invention willemerge from reading the following description, which refers to theattached figures showing preferred embodiments of the invention givenmerely by way of non-limiting examples. In these figures:

FIGS. 1 a to 1 e are illustrative diagrams in section on which thethickness and width are not to actual scale, illustrating successivestages of a method according to the invention.

FIG. 2 is a diagrammatic flow chart of a method according to theinvention.

FIG. 3 shows a field effect scanning electron microscopy (FE-SEM)photograph of a section along the growth axis of an ordered porousstructure according to the invention.

FIG. 4 shows a field effect scanning electron microscopy (FE-SEM)photograph of an ordered porous structure according to the inventionwithout the aluminium substrate but with the barrier layer, viewed fromthe side of the barrier layer.

FIG. 5 shows a field effect scanning electron microscopy (FE-SEM)photograph of the outer face of an outer surface layer according to theinvention after anodization and before mechanical abrasion.

FIG. 6 shows a field effect scanning electron microscopy (FE-SEM)photograph of the outer face of an ordered porous structure according tothe invention after mechanical abrasion, the said porous structure beingangled with respect to the anodization direction.

FIG. 7 shows a field effect scanning electron microscopy (FE-SEM)photograph of the outer face of an ordered porous structure according tothe invention, the said porous structure being angled with respect tothe anodization direction and comprising neither the aluminium substratenor the barrier layer, the said porous structure being characteristic ofnanostructuring of the “honeycomb” type.

FIG. 8 shows a field effect scanning electron microscopy (FE-SEM)photograph of the outer face of an ordered porous structure according tothe invention without the aluminium substrate and without the barrierlayer, characteristic of a nanostructuring of the “wasps' nest” type.

FIG. 1 a shows a piece 1 of aluminium or aluminium alloy serving as thesubstrate for treatment 24 by anodization and allowing an ordered porousstructure 7 according to the invention to be obtained. This aluminiumpiece 1 has at least one face, the so-called outer face 2, which issubjected to a combination of physical or chemical treatments on thepiece 1 as indicated below. The aluminium substrate used can be formed,for example, from an aluminium alloy of the series 1XXX, for example thealloy 1050A, or from refined aluminium of the 4N type (pure to 99.99%)or also of the 5N type (pure to 99.999%).

Pretreatment 18 of the Substrate

A pretreatment 18 is carried out on the piece 1 to prepare it for itsanodization 24. The purpose of this pretreatment 18 is to promote theobtaining of a thickness of ordered porous structure 7. It allows on theone hand an increase in the wettability of the piece 1 in aqueoussolution, and on the other hand a reduction in or the removal ofpre-existing defects in the surface of the piece 1. The pretreatment 18contributes towards establishing a regular contact between the piece 1and the solution for the anodization 24. By removing defects in thestructure of the piece 1, a substrate of which the outer face 2 issmooth and of which the arithmetic roughness is in particular less than5 nm is obtained. This pretreatment 18 of the piece 1 comprises asuccession of four treatments 19, 20, 21, 22.

The first treatment 19 is a degreasing of the piece 1 by means oforganic or aqueous chemical solvents. This first treatment can becarried out by steeping the piece 1 in an aqueous alcoholic solution,allowing contaminants, greases, oils or lubricants originating from theprevious methods of forming the said piece 1, for example lamination, tobe dissolved and then removed by rinsing. The piece 1 is then rinsedwith distilled water.

The second treatment 20 is a mechanical polishing allowing the roughnessof the surface of the piece 1 to be reduced and thus a smooth substrateto be obtained. In contrast to the state of the art, in which it isgenerally considered that pretexturization of the surface of thesubstrate is favourable for obtaining an ordered porous structure, theinventor has demonstrated that on the contrary it is preferable to carryout the anodization from an outer surface which is as smooth and regularas possible. In fact, defects in the structure of the substrate, whichare known to be distributed in an irregular manner over the outer face 2of the substrate, are the cause of the formation of irregular pores andof the growth of imperfectly ordered porous structures. To adjust thearithmetic roughness of the aluminium to a value of less than 5 nm,finer and finer rotating or vibrating abrasive discs are usedsequentially, and then pieces of fabric, in particular felt, impregnatedwith abrasive suspensions. Typically, a cloth impregnated with asuspension of diamond powder, the mean dimension of the diamond grainsof which is of the order of 1 μm, allows a finish suitable for carryingout the method according to the invention to be obtained. At the end ofthe mechanical polishing 20, the piece 1 is rinsed with distilled water.

The third treatment 21 comprises a heat treatment on the piece 1 withthe aim of releasing internal stresses and of increasing the size of thealuminium grains. In order to avoid oxidation of the piece 1 in thecourse of the heat treatment 21 and taking account of the speed of thekinetics of the oxidation of aluminium at a high temperature, this heattreatment 21 is preferably carried out under a non-oxidizing atmosphere,typically under a neutral or even reducing atmosphere, that is to sayunder an inert gas atmosphere, typically under a nitrogen atmosphere, oralso under a partial vacuum. The piece 1 is heated in an oven at atemperature of between 350° C. and 600° C., preferably at 450° C. Theheat treatment lasts between 0.1 h and 8 h, in particular between 0.5 hand 5 h, preferably for 1 h at an effective temperature of 450° C. undera nitrogen atmosphere.

The fourth treatment is an electropolishing 22 of the piece 1. Theobject of this is to improve the state of the surface of the outer face2 of the piece 1 which, as indicated above, must be as smooth aspossible. To this effect, the piece 1 is subjected to an electrolysisunder a voltage of between 25 V and 26 V for a duration of between 1 minand 1 h in a cell containing a bath regulated at a temperature ofbetween 20° C. and 30° C. The said bath can be an alkaline bath or anacid bath. It is, for example, a Jacquet bath. In particular, theJacquet bath is made up of a mixture of 33% by volume perchloric acidand 66% by volume glacial acetic acid, the piece 1 constituting theanode of the electrolysis. Typically, an electropolishing 22 accordingto the invention is obtained by treating the piece 1 by electrolysisunder 25 V for 2 min in a Jacquet bath thermoregulated at 20° C. Thepiece 1 is then rinsed with distilled water and subjected to thetreatment 24 by anodization immediately after rinsing.

At the end of the pretreatment 18 of the substrate, a piece 1 of whichthe outer face 2 has a low and regular arithmetic roughness, inparticular an arithmetic roughness of less than 5 nm, is obtained.

This piece 1 is used to prepare an ordered porous structure 7 by atreatment 23 comprising an anodization 24 followed by an abrasion 25.

Anodization 24

The piece 1 is subjected to a single anodization 24, in which the piece1 constitutes the anode. A single anodization 24 is understood asmeaning a treatment comprising either a single anodization stage orsuccessive anodization stages, without an intermediate stage of chemicalor electrochemical treatment of the porous structure. The anodizationconditions are preferably of the “hard anodization” type as described,for example, in the publication Lee W., Ji, R., Gösele, U. and NielschK., (2006), Nature Mat., 5; 9, 741-747 “Fast fabrication of long-rangeordered porous alumina membranes by hard anodization”.

Under these operating conditions, the speed of oxidation of thealuminium is advantageously greater than the speed of dissolution, bythe electrolyte, of the alumina formed. The anodization 24 leads to theformation of an anodic structure 35 comprising an outer surface layer 3supported by a residual aluminium layer 4.

The anodization 24 can be carried out in an electrolyte chosen fromsulfuric acid, a mixture of sulfuric acid and boric acid, oxalic acid,phosphoric acid, malonic acid, tartaric acid or also citric acid.

Typically, the use of a mixture of sulfuric acid and boric acid as theelectrolyte allows a thickness of the structure 35 of up to 300 μm to beobtained. Such a thickness of the anodic structure 35, however, does nothave an ordered porous structure 7 over its entire thickness.

To promote the formation of a high thickness of ordered porous structure7, for example, an aqueous solution of phosphoric acid at aconcentration of between 1% and 8%, preferably 8% by weight is employedin a cell of which the temperature is regulated at between −2° C. and+2° C., preferably at −1.5° C. In order to promote a homogeneous andregular growth of the porous structure, the solution is continuouslyhomogenized by agitation. The voltage applied to the aluminium piece 1is typically between 125 V and 195V.

The anodization treatment 24 is carried out for a duration sufficientfor the outer surface layer 3 to have a sufficient thickness, and forthe outer surface layer 3 to have a thickness of ordered porousstructure 7 over a part of its thickness. Under the preferred operatingconditions mentioned above, for example, an outer surface layer 3 of 130μm thickness is obtained for an anodization duration of 4 h.

The anodic structure 35 is shown in diagram form on FIG. 1 b. FIG. 1 bis merely a diagram and for illustration, and is not to scale. Itcomprises a residual non-oxidized aluminium layer 4 supporting an outersurface layer 3. The outer surface layer 3 is made up of a non-porousbarrier layer 5, also called compact layer, defining on its inner face 6the interface between the residual aluminium layer 4 and the outersurface layer 3, and on its outer face 10 the non-emerging extremity ofthe pores 8. In addition, the outer surface layer 3 comprises on itsouter face a non-ordered porous layer 11 extending from the outer faceof the outer surface layer 3 to the ordered/non-ordered interface 14with the ordered porous structure 7. The ordered porous structure 7 hasa regular juxtaposition of pores 8 empty of material in the form oflinear tubular channels of constant diameter extending axially along amain direction corresponding to the direction of anodization, orthogonalto the outer face 2 of the anodic structure 35, and dividing walls 9separating the pores 8. The dividing walls 9 additionally have aconstant thickness over the entire thickness of the porous structure 7.

Depending on the conditions of the anodization 24, the mean distancejoining the centres of two adjacent pores varies from 50 nm to 600 nmand the mean diameter of the said pores varies from 10 nm to 500 nm. Thenon-ordered porous layer 11 is formed from an irregular juxtaposition ofpores empty of material and of variable shapes, orientations anddimensions, separated by dividing walls also of variable shapes,orientations and thickness dimensions, over the whole non-ordered porouslayer 11.

In practice, it is found that the non-ordered porous layer 11superimposed on the ordered porous structure 7 partly masks andobstructs the external surface of the said ordered porous structure 7.

Abrasion 25

According to the invention, the non-ordered porous layer 11 is thenremoved from the anodic structure 35 in a manner such that at least athickness of ordered porous structure 7 is revealed.

According to a preferred embodiment of the invention, the non-orderedporous layer 11 is removed by removal of material, in particular by atleast one mechanical abrasion treatment 25. For this, a solid tool 12,such as a rotating, discoid, rigid, flat device, to the surface of whichis attached a piece 13 of fabric, in particular felt, impregnatedbeforehand with an abrasive suspension, is applied to the outer face ofthe outer surface layer 3.

The abrasive suspension is made up of an aqueous dispersion of particleswhich are insoluble in water and are characterized by their hardness aswell as by their size.

The solid particles of the abrasive suspensions are chosen from thegroup consisting of solid and abrasive materials, for example diamondand ceramics—in particular corundum.

A first part of the non-ordered porous layer 11 is removed by abrasionfrom the outer face of the outer surface layer 3 for some minutes, forexample 10 min, with an abrasive suspension formed from a suspension ofdiamond particles, the mean diameter of the said particles being closeto 1 μm. After this first abrasion stage, the surface of the porouslayer is rinsed with distilled water. In a second subsequent stage, asecond part of the non-ordered porous layer 11 is removed by fineabrasion from the outer face of the anodic structure 35 for someminutes, for example 10 min, with an abrasive suspension formed from anaqueous suspension of diamond particles, the mean diameter of the saidparticles being close to 0.25 μm.

A thickness of the outer surface layer 3 is thus removed by mechanicalabrasion 25, the said thickness being between 15 μm and 25 μm, inparticular of the order of from 17 μm to 20 μm, corresponding to thethickness of the non-ordered porous layer 11, revealing at the flat andnon-rough outer surface 16 of the piece 15 a thickness of ordered porousstructure 7.

By increasing the duration of the abrasion 25 with diamond particleshaving an mean size of 1 μm, it is possible to extend the abrasion 25 ofthe outer face of the outer surface layer 3 in a manner such that atleast a non-zero thickness of the ordered porous structure 7 ispreserved.

The piece 15 resulting from the abrasion 25 of the outer face of theouter surface layer 3 by removal of the non-ordered porous layer 11 isshown in diagram form on FIG. 1 c. This piece 15 comprises an anodizedlayer 36 supported on a residual aluminium layer 4, the said layer 36having a porosity which traverses it but does not emerge because of thepresence of a barrier layer 5 and of the aluminium layer 4.

The removal of the non-ordered porous layer 11 by mechanical abrasion 25of the surface allows the radial distribution of the pores 8 on theouter surface 16 of the ordered porous structure 7 to be preservedintact. In particular, the removal of the non-ordered porous layer 11 bymechanical abrasion 25 of the surface allows the value of the diameterof the pores 8 to be preserved unchanged at a value equal to that whichit had at the end of the anodization 24.

Adjustment 26, 30 of the Structural Properties

The piece 15 shown in diagram form on FIG. 1 c has on its outer surface16 a uniform distribution of tubular pores 8 of circular transversecross-section organized according to a hexagonal network, that is to sayaccording to a “honeycomb” configuration. The pores 8 have a circulartransverse cross-section and have, for example, a diameter of the orderof 250 nm.

In certain applications, this piece 15 can be used without othermodification, with the barrier layer 5 and the residual aluminium layer4. In other application, this piece 15 is subjected to at least one ofthe subsequent treatments 26, 30 allowing the functional properties ofthe piece 15 to be adjusted.

In a first variant of the subsequent treatment 30, the residualaluminium layer 4 is removed by electrochemical separation 31 of theanodized layer 36 and the residual aluminium layer 4. This separation 31is carried out in an agitated solution of phosphoric acid at aconcentration of between 5% and 20%, typically 16% by weight and at atemperature of between 25° C. and 35° C., typically 30° C., under analternative voltage of 30 volts for 30 min. This treatment 30 moreoverleads simultaneously to the removal of the barrier layer 5 and toopening of the pores 8, in particular on the inside face of the porousstructure 33. The piece 34 obtained has a porosity which traverses itand emerges on the two faces—outer surface 16 and inner surface 17—ofthe ordered porous structure 7, and is shown in diagram form on FIG. 1e.

In a first variant of the subsequent treatment 30, a treatment 32 canthen also be carried out by chemical dissolution, leading to widening ofthe pores 8 of the ordered porous structure 7. For this, the piece 34 isimmersed in a solution of phosphoric acid at a concentration of between5% and 16%, typically 16% by weight. The duration of the treatment 32and the concentration by weight of the phosphoric acid are chosen toincrease the diameter of the pores 8 until a value of the diameter whichis, for example, of the same order of size as the distance separatingthe centre of two adjacent pores in the ordered porous structure 7 isreached.

In a second variant of the subsequent treatment 26, on the piece 15obtained at the end of the mechanical abrasion 25 a succession of threetreatments 27, 28, 29 is carried out by selective dissolution ofconstituents of the piece 15: a first treatment 27 of controlled openingof the pores 8, a second treatment 28 of chemical/redox dissolution ofthe residual aluminium layer 4, and then a third treatment 29 ofchemical dissolution of the barrier layer 5.

The first treatment 27 comprises partial chemical dissolution of thedividing walls 9 and allows the diameter of the pores 8 to be increasedup to a value which depends on the duration of the reaction and on theconcentration by weight of the acid used. This first treatment 27 allowsperfect control not only of the diameter but also of the geometry of thetransverse cross-section of the pores 8, from a circular section up to ahexagonal section. This first treatment 27 additionally allows thediameter of the pores 8 to be modified without, however, affecting thebarrier layer 5 or the residual aluminium layer 4.

This first treatment 27 is carried out by immersing the piece 15 in asolution of phosphoric acid at a concentration of between 5 and 16% byweight at a regulated temperature, in particular between 25 and 35° C.Typically, the concentration of the phosphoric acid solution is 16% andthe temperature is 30° C. The duration of the treatment varies accordingto the desired geometry in the surface 16 of the anodized layer 36. Aduration of the treatment of 65 min leads to an ordered porous structure7 in which the pores 8 are ordered hexagonally and have a hexagonaltransverse section and a diameter of the order of 400 nm, according to a“wasps' nest” configuration. Intermediate durations of the treatmentlead to intermediate configurations between the “honeycomb”configuration and the “wasps' nest” configuration, in which the diameterof the pores varies between 250 nm and 400 nm.

The second treatment 28 by chemical or redox dissolution of thealuminium layer 4 allows the residual aluminium layer 4 to be removedspecifically. The piece 15 is immersed in an oxidizing solution atambient temperature. This oxidizing solution can be a mixture of CuCl oralso of CuCl₂ at a concentration of 0.1 mol/l and hydrochloric acid at aconcentration of 18% by weight. This immersion simultaneously causesoxidation of the metallic aluminium and reduction of the copper cations.Other redox pairs having a large difference in redox potential with thepair Al³⁺/Al can advantageously be used, in particular the pair Hg²⁺/Hg.

In a variant of the second treatment 28, an amalgam of a metal which isliquid at ambient temperature, in particular gallium or mercury, withthe aluminium of the residual aluminium layer 4 is realized. Extractionof the amalgam allows the aluminium of the support to be removed in thisway.

This second treatment 28 leads to a piece 33 having a porosity whichtraverses it, without the aluminium substrate, but which does not emergebecause of the presence of the barrier layer 5.

The third treatment 29 comprises chemical dissolution of the barrierlayer 5 by immersion of the piece 33 in a solution of phosphoric acid ata concentration of between 5% and 20%, for example of the order of 16%by weight, the temperature of the said solution being regulated between25° C. and 35° C., in particular at 30° C.

A piece 34 formed from an ordered porous structure 7 of a porosity whichtraverses it and emerges on the two faces of the piece 34 is obtained inthis way.

The piece 34, the hardness of which is low, in particular of the orderof 150 Hv, is then subjected to heat treatment in order to increase itshardness, in particular up to a value of 2,000 Hv.

EXAMPLE 1

A piece 1 of refined aluminium of 4N quality, of discoid shape, of 10⁻²m diameter and of 10⁻³ m thickness is subjected to a mechanicalpolishing 20 by means of a polisher, abrasive discs and a fabricimpregnated with a suspension of diamond particles, the mean size ofwhich decreases down to 1 μm. The total duration of the abrasion isapproximately from 20 min to 30 min. The aluminium piece 1 is thenrinsed with distilled water and placed in an oven under a nitrogenatmosphere at 450° C. for 2 h. After cooling, the aluminium piece 1 issubjected to a treatment 22 by electropolishing for 2 min in a Jacquetbath, the composition of which is 33% by volume perchloric acid and 66%by volume glacial acetic acid, regulated at 20° C. under a voltage of 25V.

Immediately after the end of the treatment 22 by electropolishing, thealuminium piece 1 is placed in an anodization cell containing an aqueousbath of 8% (by weight) phosphoric acid homogenized by rotary agitationat a speed of 37 rad/s and regulated at a temperature of −1.5° C. Thevoltage is fixed at 180 V and the duration of the anodization is 4 h.

The analysis by field effect electron microscopy of the outer face 2 ofthe outer surface layer 3 obtained after anodization 24 and beforepolishing 25 is shown on FIG. 5. This photograph shows a plurality ofpores irregularly distributed over the entire surface with a transversecross-section heterogeneous in size and in shape. It is furthermorenoted that a minority of these pores have an emerging porosity.

EXAMPLE 2

An aluminium piece 1 is prepared as described in Example 1 and subjectedto an anodization under a voltage of 185 V for 4 h.

After the anodization, the outer surface of the anodic structure 35 isremoved by abrasion 25 by means of a piece of felt impregnated with asuspension of diamond particles, the mean diameter of which is 1 μm, for10 min and then by means of a piece of felt impregnated with asuspension of diamond particles, the mean diameter of which is 0.25 μm,for another 10 min.

The aluminium piece 1 supporting the porous structure is then treatedfor 1 h with a solution of phosphoric acid at a concentration of 16% byweight, regulated at a temperature of 30° C. and homogenized by rotaryagitation at a speed of 37 rad/s.

After rinsing, the aluminium piece 1 supporting the porous structure istreated with a solution of CuCl and HCl at a temperature of 20° C. untilthe thickness of residual aluminium is totally dissolved.

The analysis by field effect electron microscopy of the longitudinalsection of the ordered porous structure 7 obtained is shown on FIG. 3. Ajuxtaposition of sections of linear tubes elongated along the directionof growth of the porous structure, the average width of which is 360 nm,can be seen.

EXAMPLE 3

An aluminium piece 1 is prepared as described in Example 1, is thenanodized under a voltage of 185 V for 4 h and finally is subjected to amechanical abrasion as described in Example 2.

After rinsing, the ordered porous structure is immersed in a solution ofCuCl and HCl, regulated at a temperature of 20° C., until the thicknessof residual aluminium is totally dissolved. A piece 33 without aresidual metallic aluminium layer 4, of a porosity which traverses itand does not emerge, and with a barrier layer 5 is obtained

The analysis by field effect electron microscopy of the surface of thebarrier layer 5 of the piece 33 is shown on FIG. 4. A juxtaposition ofnon-emerging hexagons of hexagonal cross-section regularly orderedaccording to a centred hexagonal arrangement, and of which the meandiameter of the circle describing this hexagon is 460 nm, can be seen.

On the other hand, the analysis by field effect electron microscopy ofthe outer polished face 2 of the piece 33 is shown on FIG. 6. Ahexagonal arrangement of regularly ordered pores 8 of circular section,the mean diameter of which is 300 nm, can be seen.

EXAMPLE 4

An aluminium piece 1 is prepared as described in Example 1, is thenanodized under a voltage of 180 V for 4 h and finally is subjected to amechanical abrasion as described in Example 2.

After rinsing, the porous structure 15 is treated by electrochemistryfor 45 min under a voltage of 30 V/50 Hz in a solution of phosphoricacid at a concentration of 16% by weight, regulated at a temperature of30° C. and homogenized by rotary agitation at a speed of 37 rad/s. Apiece 33 without a residual aluminium layer 4, of a porosity whichtraverses it and emerges, without a barrier layer 5 and of which thediameter of the pores 8 has been widened is obtained.

The analysis by field effect electron microscopy of the outer surface ofthe ordered porous structure 7 obtained in this way is shown on FIG. 7.A juxtaposition of pores of circular section, regularly ordered, and ofwhich the mean diameter is 240 nm, of the “honeycomb” type can be seen.

EXAMPLE 5

An aluminium piece 1 is prepared as described in Example 1, is thenanodized under a voltage of 210 V for 15 h and finally is subjected to amechanical abrasion as described in Example 2.

After rinsing, the structure 15 is treated by electrochemistry asdescribed in Example 4 under a voltage of 35 V/50 Hz for 65 min. A piece33 without a metallic aluminium layer 4, of a porosity which traversesit, without a barrier layer 5, and which emerges on the two faces of theordered porous structure 7 is obtained.

The analysis by field effect electron microscopy of the outer surface ofthe ordered porous structure 7 obtained in this way is shown on FIG. 8.A juxtaposition of pores 8 of hexagonal section, regularly orderedaccording to a centred hexagonal arrangement, of the “wasps' nest” type,of which the mean diameter of the pores is 240 nm can be seen.

1-16. (canceled)
 17. Method for the production of a porous structure inwhich an outer surface layer comprising an ordered porous structure isproduced by anodization of an aluminium substrate, whereby: ananodization treatment is carried out on a smooth aluminium substratewith a duration sufficient to allow at least a thickness of orderedporous structure to be obtained, a part of the thickness of the saidlayer formed by anodization is then removed by mechanical machining,this part of the thickness extending from the outer surface of the saidlayer formed by anodization, while maintaining at least a non-zerothickness of the ordered porous structure and in a manner such that thisordered porous structure forms the free outer surface of the residuallayer.
 18. Method as claimed in claim 17, whereby the said part of thethickness is removed by mechanical abrasion.
 19. Method as claimed inclaim 18, whereby the mechanical abrasion is carried out by means of apiece of fabric impregnated with a suspension, a so-called abrasivesuspension, of a powder in an aqueous phase, the said powder comprisingat least a mineral chosen from the group of abrasive minerals. 20.Method as claimed in claim 18, whereby the mechanical abrasion iscarried out by a plurality of successive abrasion stages, each of thesaid successive abrasion stages being carried out by means of anabrasive suspension, the abrasive suspensions of each of the successiveabrasion stages being chosen in a manner such that a granulometry whichdecreases from one stage to the other is achieved.
 21. Method as claimedin claim 20, whereby each abrasion stage of the plurality of successiveabrasion stages is carried out by means of a piece of fabric impregnatedwith an abrasive suspension, the said piece of fabric being applied tothe surface of a rigid support chosen from the group formed by avibrating support and a rotating support.
 22. Method as claimed in claim18, whereby the mechanical abrasion is carried out by a first abrasionstage by means of a piece of felt impregnated with a suspension ofdiamond, the mean granulometry of which is between 0.8 μm and 1.5 μm, inparticular of the order of 1 μm, and then by a second abrasion stage bymeans of a piece of felt impregnated with a suspension of diamond, themean granulometry of which is between 0.2 μm and 0.4 μm, in particularof the order of 0.25 μm.
 23. Method as claimed in claim 17, whereby apart of the thickness of the outer surface layer (3) is removed, thethickness of the said part of the thickness of the outer surface layerbeing between 15 μm and 25 μm—in particular of the order of from 17 μmto 20 μm.
 24. Method as claimed in claim 17, whereby an anodizationtreatment is carried out on a smooth aluminium substrate with a durationsuitable for obtaining an outer surface layer having a thickness ofbetween 25 μm and 300 μm, in particular between 100 μm and 200 μm. 25.Method as claimed in claim 17, whereby a single anodization treatment iscarried out on a smooth aluminium substrate, the said treatment having aduration of between 1 h and 12 h, in particular of the order of 4 h. 26.Method as claimed in claim 17, whereby a single anodization treatment iscarried out on a smooth aluminium substrate for a duration suitable forthe thickness of the ordered porous structure formed by anodization tobe between 1 μm and 150 μm.
 27. Method as claimed in claim 17, wherebythe anodization is carried out in an aqueous solution of electrolytechosen from the group formed by aqueous solutions of oxidizing acids—inparticular sulfuric acid, a mixture of sulfuric acid and boric acid,oxalic acid, phosphoric acid, malonic acid, tartaric acid and citricacid.
 28. Method as claimed in claim 17, whereby the anodization iscarried out in an aqueous solution of electrolyte, the composition ofwhich is suitable for providing an ordered porous structure, the poresof which have a diameter of between 10 nm and 500 nm, in particularbetween 100 nm and 200 nm.
 29. Method as claimed in claim 17, wherebythe anodization is carried out at a temperature of between −2° C. and+2° C.—in particular of the order of −1.5° C.
 30. Method as claimed inclaim 17, whereby the anodization is carried out under a voltage ofbetween 19 V and 240 V—in particular between 125 V and 195 V, with anaqueous solution containing phosphoric acid as the electrolyte. 31.Method as claimed in claim 17, whereby immediately after having removedthe said part of the thickness, the non-oxidized aluminium substrate anda part of the non-porous thickness of the said layer are removed topreserve only the ordered porous structure.
 32. Method as claimed inclaim 17, whereby a chemical treatment is then carried out on theordered porous structure which is suitable for increasing the diameterof the pores of the said porous structure.
 33. Method as claimed inclaim 19, whereby the mechanical abrasion is carried out by a pluralityof successive abrasion stages, each of the said successive abrasionstages being carried out by means of an abrasive suspension, theabrasive suspensions of each of the successive abrasion stages beingchosen in a manner such that a granulometry which decreases from onestage to the other is achieved.